1. Technical Field
The invention is related to ferroelectric optical computing devices, including such optical computing devices as (a) memory cells and (b) logic switches, and in particular to reading out and evaluating such devices nondestructively at very low optical power levels.
2. Background Art
An optical computing device can be made of a ferroelectric material such as a lead zirconate titanate (hereinafter, "PZT") thin film. As employed in this specification, the term "optical computing device" includes memory cells as well as logic switches. The class of "ferroelectric" belongs to a group of materials which are non-centro symmetric crystals, of which PZT is one example. Such materials exhibit two stable opposite electrically polarized states, in which the ferroelectric crystal has an electrical polarization field along a principal or "C" axis of the crystal, the polarities of the two states being opposite. In the case of a PZT ferroelectric crystal, the polarization arises from the displacement of a zirconium or titanium atom in the center of each crystal unit along the crystalline C-axis in either one of two possible directions to either one of two stable locations. The crystal is put into one of these two states by applying along the crystalline C-axis a large external electric field whose polarity is in the direction of the desired crystal state. This may be done by either applying a D.C. electric field across a pair of electrodes displaced along the crystalline C-axis or by illuminating the crystal with light polarized along the crystalline C-axis in the desired direction with a wavelength corresponding to an energy equal to or exceeding the band gap energy of the crystal. One such device disclosed by Thakoor, "High speed, nondestructive readout from thin-film ferroelectric memory," Applied Physics Letters, Volume 60, Number 26, pages 3319-3321 (29 Jun. 1992), is illustrated in FIG. 1 of the present specification. A ferroelectric crystal thin film layer 10 having a thickness between 0.15 and 0.3 microns is sandwiched between a bottom platinum electrode 15 lying on a silicon dioxide passivation layer 20 of a silicon substrate 25 and a top platinum electrode 30. At least nearly all of the crystal units within the thin film layer 10 have their C-axes oriented in the same direction, this direction being perpendicular to the plane of the thin film crystalline layer 10, as indicated by the arrows inside the layer 10 in FIG. 1. With this perpendicular orientation of the crystalline C-axis, the polarization of the crystal layer 10 may be set by applying a voltage of the desired polarity across the two electrodes 15, 30.
The polarized state of the optical computing device of FIG. 1 is sensed or "read-out" by applying to the top side of the device linearly polarized radiation 35 of sufficient power density and a suitable wavelength corresponding to a photon energy below the band gap energy of the ferroelectric crystalline thin film layer 10. If the thin film layer 10 is a PZT crystal, then the power density of the linearly polarized radiation 35 is about 20 milliWatts per square micron and the wavelength is about 532 nanometers, which is available from convention laser sources. For a pulsed laser source, the electrical (current) response across the electrodes 15, 30 is characterized by the waveform of FIG. 2A if the thin film layer 10 is in one of its two polarized states and by the waveform of FIG. 2B in the other polarized state. Comparing the waveforms of FIGS. 2A and 2B, the responses of the two polarized crystalline states are electrical currents of opposite polarities. Thus, the optical computing device of FIG. 1 can switch between binary logic states to serve as a computer logic switching device, and can store and read-out such logic states in non-volatile fashion to serve as a non-volatile computer memory. Typically, the radiation 35 is a laser beam having a beam diameter less than the width of the top electrode 30 so that the entire beam 35 is incident on and confined to the top electrode 30.
One limitation of such devices is that the required power to "read-out" the polarization state stored in the crystalline thin film layer is relatively high (e.g., 20 milliWatts per square micron for PZT material). This limitation is problematic because it imposes a relatively high power consumption level in such devices and poses a heat dissipation problem in integrated circuits constituting large arrays of such devices.
Another limitation of said devices is that they tend to be susceptible to "imprint" failures. An imprint failure occurs whenever a device cannot be changed from its initial polarization state, or, if it is changed, tends to drift from the new polarization state back to the initial polarization state. This can occur whenever the device is left in a particular polarization state for a relatively long period of time (e.g., months), or temperature cycling of the device. It is thought that imprint failure arises from charge mobile defects that accumulate preferentially in one direction with extended time/temperature/voltage cycling. So, under the influence of the internal field of these defects, the switching of the polarization state of the ferroelectric film by an applied external voltage is inhibited, impeded and unstable.
Another problem of such devices is fatigue, which is a reduction in the polarizability of such devices as a result of voltage cycling. Fatigue and imprint failure are problems that are common to all ferroelectric based optical computing devices, including memories as well as logic switches. Fatigue is thought to occur because of accumulation of mobile defects and free space charge that give rise to local fields which screen the applied external field, thereby reducing polarizability of the ferroelectric thin film.